66 Henry Riddell on 



sion indeed of the difficulty, due in large degree to his own 

 failure to find a solution, but no idea at all that it is contrary to 

 nature and common sense to expect such a solution. 



Thus, while a strong body of opinion existed about the year 

 1700 completely opposed to the theories of the perpetual motion 

 seekers, yet for many years these ideas remained fixed m the 

 minds of many mathematicians and philosophers. 



Among these may be mentioned Gravesande, a physicist ot 

 eminence in his day, and John Bernouilli, while in a continental 

 scientific journal a letter is found from Sir Humphrey Davy, in- 

 forming his correspondent that a certain machine of the perpetual 

 motion class had been examined by Playfair and Captain Kater, 

 who were of opinion that it had solved the problem. There were 

 many others, but there is only space to refer to one or two. 



In Gravesande's letter to Newton he pleads that perpetual 

 motion is an inevitable consequence of the fact that the force of 

 impact of a moving body is measured by the product of its mass 

 upon its velocity. Leibniz also asserted that if the force of impact 

 were proportioned to the velocity instead of to the square of the 

 velocity, as he taught, perpetual motion would be possible. Thus 

 he assumed as an axiom this impossibility, and used the axiom to 

 prove that the true value of the force of impact was not proportional 

 to the velocity but to the square of this number, just as at a much 

 later time the same axiom was to be used to prove the theorem as 

 to the maximum efficiency of the heat engine. 



In his work on Natural Philosophy, published some time 

 later than his letter to Newton, Gravesande alters his position. 

 He has, in the meantime, convinced himself by many experi- 

 ments that Leibniz' view of impact is the true one, and that 

 therefore the reason he previously gave did not hold. He cannot, 

 however, see that perpetual motion is thereby proved impossible. 

 " We do not know enough about the laws of Nature," he says, 

 "and it does not seem that we ever shall." 



This is the essence of the argument of a host of inventors 

 since that day, It is arrogance, say they, to assume such a know- 

 ledge of the laws of Nature as would allow us to lay down dog- 



